The present invention relates to the combining of video images, and more particularly to a method of producing a composited video image based on depth by sorting depth signals associated with the video images from farthest to nearest before inputting of the video images to a serial combiner.
Previous methods of combining video images, as in the Kaleidoscope Digital Video Effects product manufactured by The Grass Valley Group, Inc. of Grass Valley, Calif., United States of America, perform in a serial manner. Two images are combined, and the resulting image becomes a new image that can then be combined with a third image. U.S. Pat. No. 4,851,912, issued to Jackson et al on Jul. 25, 1989 entitled "Apparatus for Combining Video Signals", describes such a combination process in which the two video images are combined on the basis of a priority signal as shown in FIG. 2 of that patent. This priority signal may be made up of a combination of the depth signals from the foreground and background video images. The priority signal may be passed from stage to stage, as are the video and key signals associated with the video image, to provide input to the generation of the priority signal for the next combination stage. However if two video images are combined with priority based on depth and then a third depth based video image is combined in a serial fashion with the previously combined video images, improper priorities may result.
A type of parallel combining is disclosed in U.S. Pat. No. 4,684,990 issued Aug. 4, 1987 to Leslie J. Oxley entitled "Method and Apparatus for Combining Multiple Video Images in Three Dimensions." FIGS. 5A and 5B of the Oxley patent show a four channel depth to "coefficient" converter. If this configuration is increased to a five channel converter, for example, each stage requires additional circuitry as well as the addition of another stage. Such additional circuitry would include one more depth circuit 36, ten more subtractors 52, ten more weighting circuits 54, ten more minimum circuits 56, one more clip circuit 58, one more multiplier 60, ten more inverters 62, one more final minimum circuit 64 with an additional input for all, one more output multiplier 72 and an added input for summing circuit 68. Adding just one additional layer therefore adds large amounts of circuitry with the attendant additional cost.
What is desired is a method of combining a plurality of video images based on depth that maintains proper depth order between the video images in a simple, cost effective manner.